According to Explorer Jack, Isidor is an acquaintance of his, and who now resides on the second floor of the Wizards' Tower, but 'has a memory like a sieve' due to his experimenting with magic all day.
If you have not yet started the quest and speak to Isidor, you will learn that he is a wizard who "works on portals and teleportation" and could teleport you anywhere, in theory, if he has a "a precise means of knowing where, like a phrase or something" but is working on something else right now and will be unable to teleport you to the Land of Snow.
According to Explorer Jack, he is awfully forgetful and easily distracted, and for that reason you have to remind him of the teleport spell every time you want to go to the Land of Snow.
Isidor has a phobia of sleds. If you try to bring a sled to the Land of Snow he will say: "I've a phobia of sleds, you see. If I were to teleport you with one, who knows where you'd end up?" This is probably due to technical reasons.